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    <title>Document</title>
    <script>
            var a = 100;
            var b;
            b = 200;
            console.log(a,b);
            c = 300;
            console.log(c);
            var d;
            console.log(d);
            // console.log(e); //error
            //常量
            const PI = 3.14;
            console.log(PI);
            // PI=3.1415926; //常量不能更改
    </script>
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